Composting is the purposeful biodegradation (rotting) of organic matter, such as garden and food waste.

The Technical Stuff

Compostable material rots under certain conditions into humus that is very valuable in the garden. This is performed by micro-organisms, mostly bacteria, but also yeasts and fungi. Also contributing to the process are soldier flies, fruit flies and fungus gnats. The size of the material affects its compostability. Reducing particle size can speed the process. Large pieces of hardwood for example may not compos under normal conditions, but sawdust form the same type of wood may do. Biodegradable materials are not suitable for home composting.

Composting organisms require four equally important things to work effectively:

Carbon ("C" or carbohydrates), for energy - the microbial oxidation of carbon produces the heat. High carbon materials tend to be brown and dry.

Nitrogen ("N" or protein), to grow and reproduce more organisms to oxidize the carbon. High nitrogen materials tend to be green and wet (or colorful, like fruits and vegetables).

Oxygen, for oxidizing the carbon, the decomposition process.

Water, in the right amounts to maintain activity without causing anaerobic conditions( lack of air )

A certain mix of these elements will provide the bacteria with the nutrients to work at a rate that will heat up the pile. In that process water will be released as vapor ("steam”) and the oxygen will be quickly be used up. The hotter the pile gets, the more often added air and water is needed so you have to “manage” the pile. The air/water balance is vital to maintaining high temperatures until the materials are broken down. At the same time, too much air or water also slows the process, as does too much “brown” or “green” material.

Mixing equal parts by volume gives more-or-less (mais ou menos) the ideal Brown/Green mix, you are not looking for precise proportions just keep this in mind. This is not an exact science so don’t spend your whole life on the compost heap, just a bit of time getting it right will be well rewarded with lots of good quality FREE compost.

Paper and card board, is not recommended as both the ink and paper contain materials that are not biodegradable and paper will decompose very slowly interfering with the composting process.

Composting in Central Portugal

Here in Central Portugal we have such extremes of temperature and weather that Active Composting is the only really successful method of composting, and can give good quality compost within 2-4 months. This means that if start off in February then by the end of May you could have your first batch ready for using.

It used to be difficult to buy plastic compost bins in Central Portugal but you can now find them in most DIY stores and in Lidl in the Spring. The other method is to build wooden compost bins using rough sawn timber. You should be able to get the off-cuttings from tree trunks at your local timber yard, this works very good for the sides as they allow some air flow and look very rustic too, or even old pallets.

A compost bin of about 1cubic meter is best. This provides enough mass to build up heat but also allows air in. It is better to have 2 composting bins, so that when the first phase is complete you cover the pile with an old piece of carpet or strong polythene for it to mature and start a new pile.

Covering is necessary in Central Portugal for either summer to keep moisture in or through winter to keep the rain out and keep it warm. In Spring and Autumn it is not as important, and this is a good time to do Active Composting, as it is not too wet or too hot.

Active Composting

Mixing the materials as they are added increases the rate of decomposition, as does reduced particle size (i.e., chopped, shredded), or materials being added in alternating layers, about 15 centimeters (6 in) thick. Keeping a ‘carbon pile’ handy for covering and mixing with fresh wet additions is simplest if you have the space. Other additions are not necessary, although some sprinklings of good garden loam as a first pile is built will aid more rapid working by adding in “good” soil bacteria and provides grit to help earthworms digest the compost. Adding some of your first completed compost to your next batch perpetuates this cycle.

If the pile is built in a short period, and has a good mix of materials and a coarse structure, with about 50% moisture ("like a squeezed out sponge"), the temperature should rise within days to as high as 60 °C (140 °F). When the temperature begins to fall, more air is needed, usually added by turning the pile with a simple garden fork is good for this and moisture may be added at the same time. The center of the pile heats up the most, so regular turning/mixing is needed to ensure all material spends some time in the hottest area. Turning or other aeration is usually needed about every 6–10 days to maintain the highest heat levels until the material is fairly uniformly broken down. A pile that has been maintained at peak temperatures may be ready for maturing in as little as 30 days, but rarely sooner than 60 days. Another 30–60 days of maturing should see you with usable compost.

When the matured material has a dark brown crumbly appearance and the smell of rich damp earth, it is ready to use.